1. Field of the Invention
This invention is generally related to methods and apparatus for electrostatic treatment of water streams for improving particulate dispersions. In particular, the invention describes a device of novel construction that allows reliable operation at much higher voltages, and correspondingly with higher efficiencies, than previously reported.
2. Description of the Related Art
All water systems are commonly subject to reduced efficiency and failure as a result of scaling and clogging by solid particles suspended in the aqueous medium. As a means for reducing these problems, the concept of treating the water by inducing an electrostatic field across it has been known for many years and many devices have been utilized with varying success, both for industrial and domestic applications.
Solids accumulation is a particularly severe problem in solvent extraction circuits, such as in mining operations and other large-scale industrial applications, wherein suspended solids bind to form masses of sludge that decrease the efficiency and inhibit the operation of the equipment. It has been found that the degree of accumulation depends on the particulate content of the water, the operating conditions of the circuit, and the type of process involved (such as, for example, a particular leaching technique). The sludge consists principally of silicate mineral matter bound by organic components dispersed in water. Most solids enter the system in suspension and some precipitate while passing through, causing scaling and accumulation of sludge. Regardless of the particular nature of the water circuit, the presence of accumulated sludge invariably results in increased operating expenses and additional capital costs associated with control efforts.
For some time researchers have known that the surface charge of particles in suspension can be altered by exposure to high-potential electrostatic fields. A particulate dispersion is improved as a result of elevated levels of induced like charges on the surface of sub-micron particles that yield mutual repulsion of the particles. Similarly, the particles may become attracted by exposure to various levels of charge intensity that yields reduced repulsion among them. This phenomenon forms the basis for well known applications, such as in electrostatic precipitators, photocopiers, and certain paint spraying techniques. U.S. Pat. Nos. 3,585,122 (1971) to King and No. 4,073,712 (1978) to Means et al. describe water-treatment electrostatic systems of the type addressed in this disclosure. Means et al. discuss in detail many of the fundamental principles underlying the effects of electrostatic fields on particle suspensions in water and describe a device for efficiently inhibiting scale formation in a hot-water system. The patent discloses an analytical approach to the design of water treatment apparatus having two tubular electrodes mounted in concentric spaced relation to form a series of three capacitors (two dielectric materials surrounding a body of water therebetween) subjected to a high-voltage DC electrostatic field. The preferred dielectric materials consist of thin layers of polytetrafluoroethane (PTFE, also known commercially under the registered trademark TEFLON) and aluminum oxide, each having a dielectric coefficient sufficient to ensure that most of the electrostatic field intensity is applied across the body of water. Similarly, in U.S. Pat. Nos. 4,545,887 (1985) and No. 4,902,390 (1990), Arnesen et al. describe electrostatic electrodes for a storage tank of a water system also based on the use of PTFE heat shrunk over a conductive tube.
In all devices found in the prior art, the water to be treated is subjected to an electrostatic field created by insulated electrodes arranged to produce a series of capacitive layers between them. Given the very high voltages applied to the electrodes (the device described by Means et al. is most commonly operated at about 10,000 DC volts), the integrity and strength of the insulation between the water and at least one of the electrodes is crucial for the continued operation of a system. Any breakdown of the dielectric layer causes a short through the water body and the inevitable shutdown of the system. Therefore, all known devices are constructed so as to ensure the integrity of the dielectric material used to insulate the usually positive electrode. This is achieved in all cases by enveloping a tubular metallic electrode in a Teflon.RTM. sleeve that is heat shrunk around the outer surface of the electrode and by sealing each end of the resulting insulated electrode with protective dielectric bushings. Thus, this process provides a seamless insulating layer of Teflon.RTM. around the metallic electrode and ensures intimate contact between the two materials. Such intimate contact is very important because any air space left between the metal and the dielectric, such as by blisters or bubbles in the dielectric layer, causes electrical arcing between the two that eventually perforates the Teflon.RTM. layer, shorts the electrode to the water body, and greatly reduces the electrostatic efficiency of the device. Moreover, a large air space would form yet another dielectric layer within the system, which is undesirable because of the very low capacitance of air that would greatly reduce the overall capacitance of the system.
In all cases, the objective of an electrostatic device is to apply the maximum electrostatic field across the fluid being treated. This goal is based on the assumption that the surface charges of organic and inorganic particles in the water are the responsible mechanisms promoting agglomeration and aggregation of sludge mass. It follows that anything done to alter the charge differentials that promote bonding of suspended particles with organic compounds serves to establish a more stable dispersion of solids. Since, for a given type of apparatus, the electrostatic field across the water medium is proportional to the potential applied to the system, it is desirable to apply as high a voltage as possible within the tolerances of the apparatus. Higher voltages have been found to be more effective, at times essential, for treating waters with high dissolved or suspended solid concentrations (such as with more than 1,000 ppm total dissolved solids) which have been shown to be totally unaffected by the apparatus of the prior art. The inefficiency of these devices is explained by the fact that the effective dielectric constant of water increases with increased content of dissolved solids. The result is a reduction of the voltage gradient in the water that can drop below the critical level necessary for producing a successful colloidal dispersion, which is the mechanism for scale or sludge deposit reduction. For a given water quality and flow rate, there is a critical field intensity below which no electrostatic effect is noted.
The devices of the prior art are limited in their application by twofold problems. Because of its well-known physical properties, PTFE material is not suitable for adherence to the surface of metal conductors other than by the heat-shrink process described in the referenced patents. Any attempt to cover an electrode with Teflon.RTM. by a process other than heat-shrinking (such as would be required for an electrode having a non-cylindrical shape) would necessarily compel the formation of seams and connections that would be very hard to achieve and prone to breakdown during use. In addition, because of the material's non-stick properties, it would be very difficult to avoid the formation of air spaces between the metal and the dielectric surfaces. Accordingly, the preferred structure of such electrostatic devices is cylindrical, as described above, wherein each end of the insulated tube is sealed by means of separate dielectric bushings. Under normal stresses of operation, the connection between the tube and these end bushings has been the source of leaks that allow the water medium to come into contact with the high-voltage metallic tube and cause a complete system breakdown. U.S. Pat. Nos. 4,024,047 (Clark et al.) and No. 4,199,430 (McMahon) provide some solutions toward improving the water-tight connection between the electrostatic tubes and the end caps, but still require the use of separate end components. Therefore, it would be desirable to have an electrostatic device of such physical configuration that potential sources of leaks between the water body and the high-voltage metal core are minimized.
Another problem is related to the thickness of the dielectric material utilized in the prior art. In order to optimize its capacitance, the layer of Teflon.RTM. used to coat the positive electrode is kept to a minimum (Means et al. disclose five to twenty-five thousands of an inch as the preferred thickness). This causes the dielectric layer to be more vulnerable to imperfections of construction that might cause arcing or other operating stresses that could result in interruption of insulation. As a result of these constraints, the devices of the prior art are not suitable for efficient and dependable operation at voltages higher than approximately 10,000 volts, beyond which they quickly experience breakdowns. This characteristic prevents their utilization for large water-treatment systems and for waters containing high concentrations of dissolved solids, both of which require very high electrostatic potentials applied across the water body in order to process high-volume throughputs.
Because of these practical problems, the concept of applying an electrostatic field to a water suspension to effect its physical characteristics has been exploited only in relatively small water treatment systems (i.e, low throughput and/or low solid content), such as described in the referenced patents. Therefore, there is still a need for an improved electrostatic device that is operable at very high voltages with reliability and safety. In particular, there is a need for an electrostatic device that is not susceptible to total breakdown as a result of breakage or interruptions in the dielectric integrity of the material.